Film writing system with closed loop control for electronic images

ABSTRACT

An imaging system for generating a image on film from video image data incorporates feedback control techniques to provide control of film density. The system includes variable gain amplifiers responsive to light intensity output of a film writing device for maintaining the output at a desired set level. In addition, the system includes apparatus and a method for monitoring the density of film images produced by the system and for utilizing the monitored values to modify the system transfer function in a direction to effect a change in the film density towards a desired value.

BACKGROUND OF THE INVENTION

The present invention relates to medical x-ray systems and, moreparticularly, to a system for producing uniform x-ray film images fromelectronic images.

Currently available film writing systems are subject to variations intheir light output which give rise to variations in film density. Suchfilm density variations may effect the ability to accurately diagnosemedical problems using x-ray film analysis. The problems of film densityvariation are also aggravated by variations in the film itself and inthe film processing. Since film processing involves a chemical treatmentand reaction, the chemicals become contaminated with repeated useresulting in film density variations as a function of time and use.

Compensation for film density variations in the currently availableprocessing systems have relied on trial and error techniques. Typically,a known reference image signal is provided to the system and a filmexposed and developed. An operator then evaluates the image density onthe film and adjusts the gain of the system in a direction to compensatefor any deviations in film density from a desired norm. This process isrepeated until a satisfactory film density is attained. Unfortunately,instabilities in the control system for the light source and changes inthe film processing result in early deterioration of film quality thusrequiring a repeated alignment procedure by trial and error adjustment.

It is an object of the present invention to provide an improved systemfor converting electronic images to film images.

It is another object of the present invention to provide an improvedsystem for converting electronic images to film images whichautomatically compensates for variations in light output.

It is a further object of the present invention to provide an improvedsystem for converting electronic images to film images which includescompensation for variations in film density.

DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference may behad to the following detailed description taken in conjunction with theaccompanying drawing in which:

FIG. 1 is a simplified functional block diagram of a control system inaccordance with the present invention;

FIG. 2 is a simplified diagram of a film density metering system; and

FIG. 3 is a functional block diagram of a feedback control system forcontrolling write intensity output in accordance with the presentinvention.

FIG. 4 is a functional block diagram of an alternate embodiment of thesystem of FIG. 3 in which the density control function is incorporatedin the write intensity feedback control loop;

FIG. 5 is a block diagram of an automatic density control apparatusincluding an alternate form of calibration film holder cassette for usewith the embodiment of FIG. 4; and

FIG. 6 is an illustration of minimum and maximum video intensitycalibration signals.

SUMMARY

In accordance with one embodiment of the present invention an electronicfilm writing system and a film processing system are combined into acontrol system including a first control loop to regulate light outputof the film writing device and a second control loop to compensate forvariations in developed film density. In the film writing system a lightsource such as a cathode ray tube (CRT) or laser provides a light outputwhich can be used to impress an image on a film. The light soruce isdriven by a variable gain, adjustable offset amplifier at a level toproduce sufficient light to properly expose a film. In order to maintainthe light output at a constant regulated level, a light sensor isconnected in a light path adjacent the light source and provides acontrol signal representative of light intensity. The control signal isapplied to the driving amplifier as a feedback signal to adjust thedrive signals supplied to the light source in a direction to compensatefor deviations from a desired or set level.

The film density is controlled by a second feedback loop whichincorporates a film density measuring system to provide a density signalrepresentative of film density. The density signal is also applied as again varying and offset signal to control the magnitude of a video imagedata signal coupled to the driving amplifier for the film writingdevice. Each of the control system feedback loops are synchronized toselected cycles of operation of the system in order to avoid mutualinterference. In an alternate embodiment, the density control isincorporated in the intensity control loop.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is shown a simplified block diagram of anelectronic image processing system including a film processing systemincorporating a control system in accordance with the present invention.Although the system will be described in conjunction with a preferredembodiment for the control of images on x-ray film, it will beappreciated that the invention may be applicable to other filmprocessing systems. The overall system function is to convert image datasupplied to the system at point 10 into viewable images on eitherpositive or negative x-ray film at point 12. In effecting this result,it is necessary that the data converted to an image on x-ray filmaccurately represent the image data supplied at point 10. It is alsoimportant that the image represented on the x-ray film be of a highquality to enable accurate diagnosis of any medical problem appearing onthe film. In achieving this last function, the conversion system must becapable of maintaining uniform film density and yet provide for aminimum amount of setup and adjustment time.

The image data supplied at point 10 may be either in analog or digitalform. If the image data is supplied as an analog signal, the block 14may comprise a variable gain linear or piecewise linear amplifier. Inthe case of image data supplied in digital form, the block 14 maycomprise a digital memory device and associated processing circuitry toenable the image data to act as an address or a pointer to identifyparticular values stored in memory in what is well known in the art as alook-up table. The block 14 may appropriately be characterized as atransfer device having a predetermined transfer function for convertingthe input signal V_(IN) to an appropriately corresponding output signalVO.

The output signal developed by the block 14 is connected through anelectronic switch 16 to a further block 18. The block 18 represents asecond amplifier which also has an adjustable gain and in addition, iscapable of having a zero offset which is variable in response to acontrol signal. As can be seen from the graph of the voltage input toblock 18 labelled VO and the voltage output labelled V_(D), theamplifier 18 provides an additional gain and offset for the V_(O)signal.

The V_(D) signal produced by the block 18 is coupled to appropriateinput terminals of a film writing device which may be a cathode ray tube(CRT) or laser, both of which are well known in the art. As will beappreciated by the shape of the graph shown in the block 20, the CRT orlaser characteristic tends to be non-linear with respect to the inputdrive signal, i.e., the brightness of the generated light beam is anon-linear function of the amplitude of the input drive signal.

Continuing with the block diagram, there is shown a final block 22 whichrepresents the film developing and processing in order to develop thefinal film image at point 12. As will be appreciated, the output signalindicated as produced by block 20 is in reality the light output of theCRT or laser which is used to expose the x-ray film prior to developing.The effect of any given writing intensity, I, is to produce a filmdensity, D, on the exposed film. The film density versus exposureintensity characteristics shown in the block 22 is the well knowncharacteristic curve of the film and the processing system.

In accordance with the present invention, the control system includestwo feedback loops, a first of which monitors the light intensity fromthe film writing device 20 and provides a feedback signal to thevariable gain amplifier 18 in order to maintain the light output of thefilm writing device at a desired level. The second feedback control loopmonitors the density of the exposed and developed x-ray film andprovides a feedback signal to the variable gain amplifier block 14 whichadjusts the magnitude of the image data signal supplied to the block 18in order to drive the film density in a direction to correct for anydeviations from a desired density. As will be appreciated, there isprovided a calibration mode in which the initial values of lightintensity output from the film writing device may be establishedfollowed by a subsequent step of providing a reference data signal whichallows the film density to be adjusted to a desired value. In thecalibration of the film writing device 20, the switch 16 is placed in aposition such that a voltage reference signal V_(R) ' is applied as aninput signal to the amplifier 18. Obviously, the use of the V_(R) 'signal is to assure that a known reference is applied for calibrationpurposes. A signal I_(R), which is summed in summing block 24 with afeedback signal representative of write intensity or light output of thefilm writing device 20, is simply an adjustment signal which can bevaried in order to adjust the light output to a desired level.Similarly, film density is established by providing a known referencesignal V_(R) as an input signal to the block 14 while monitoring thedensity of a film produced at point 12. The feedback loop providessignals indicative of film density which are then summed in summingblock 26 with a variable signal D_(R) which can be adjusted to obtainthe desired film density at point 12.

It will be apparent that the closed loop control of film densityrequires that the film first be exposed with a known gray scale of writeintensities and thereafter developed. Subsequently, the densitiesproduced on the film in response to the gray scale exposure must be readto determine whether the write intensities must be adjusted such thatthe desired film densities are achieved. In order to accomplish thesefunctions, it is necessary that the write intensity output of the CRT orlaser device 20 be adjusted to provide an initial known light intensity.Once the film has been exposed and then developed, it can be returned tothe writing point in front of the device 20 and the calibrated lightoutput of the writing device 20 can be used to read the actual filmdensity. One form of a reading process is illustrated in FIG. 2. As isshown in the regulated light beam from the writing device 20 is firstpassed through a calibrated sensitometric block 28 of a type well knownin the art. The sensitometric block 28 is mounted adjacent the filmwriting device 20 in a position abutting the point at which film isinserted to be exposed to the light from the film writing device 20. Inthe calibration mode the film is placed in position so that the lightbeam passes through both the sensitometric block 28 and the film 30. Thefilm 30 also includes an exposed sensitometric strip 32 normallypositioned along an unused edge of the film. The sensitometric strip iscreated by the calibration drive signals applied to the film writingdevice 20 during the calibration cycle. Creation of a sensitometricstrip on an unused edge of x-ray film is well known in the art and willnot be described in further detail. During the calibration process ofthe present invention, the regulated light beam from the writing device20 is scanned in a vertical direction along the edge of the film strip30 first passing through each step of the sensitometric block 28 andthen through each of the gray scale blocks on the exposed sensitometricstrip 32. A photodetector 34 located behind the film strip 30 senses thelight from the film writing device 20 which passes through thesensitometric block 28 and sensitometric strip 32 and provides outputsignals proportional to the magnitude of the light impinging on it. Thephotodetector 34 preferably comprises a single photodector which scansin synchronism with the regulated light beam from the film writingdevice 20 so as to receive the light beam as it scans down the edge ofthe film 30. The output signals developed by the photodetector 34 areproportional to the amount of light passing through each section of thesensitometric block 28 and sensitometric strip 32. Since thesensitometric block 28 is a highly calibrated block, the initial portionof the scan is be used to calibrate the photodetector 34. As the lightbeam thereafter passes through the sensitometric strip 32, thephotodetector 34 will provide output signals representative of thedensity of the image on the film 30. It should be noted that the imageforming the exposed sensitometric strip 32 is created by a step voltageV_(R) applied to the first amplifier 14 of FIG. 1. The use of a stepvoltage V_(R) to produce the sensitometric strip 32 is well known inart. Thus, it will be seen that by using the calibrated light outputfrom the film writing device 20, film density can be controlled byperiodically creating a sensitometric strip 32 on an edge of a film 30and inserting the developed film in front of the writing device to allowthe photodetectors 34 to compensate the amplifier 14 for any deviationsin film density from the desired value. Although the preferred method ofcalibration of film density is through use of the regulated light beamfrom the film writing device 20, it will be appreciated that an externalcalibrated light beam could be used for reading the film densities.

By performing the calibration sequence described above, it will beappreciated that the data supplied to the transfer device 14 can be usedto establish a table of film density as a function of the referencevoltage V_(R). In the case of a look-up table, i.e., a digital memorysystem for processing digital image data, the transfer function will bemodified to provide an output voltage for any given input voltagecommand necessary to produce the desired film densities. In the look-uptable format, the system has a greater capability of compensating fornon-linear variations in the overall image conversion process. Thelook-up table can be constructed to have as many reference densitysettings as there are reference input voltage levels. By so doing, filmgamma correction and/or corrections for the non-linearities of the filmwriting device 20 can be accommodated. However, the use of image data inan analog form provides a much simpler approach since the transferdevice 14 may comprise a linear operational amplifier circuit withvariable offset and variable gain. The gain and offset can then beadjusted so that the minimum and maximum film density signals producethe desired result and the other signals falling between the minimum andmaximum levels would then produce an output signal varying linearly orpiecewise linearly between the minimum and maximum values.

In FIG. 3, there is shown in more detail the calibration and controlloops for regulating the writing intensity of writing device 20. Beforebeginning the detailed description, it might be noted that the imagedata supplied at terminal 10 is in the form of a video signal having atypical television format. In particular, the image data is of a typewhich can be displayed by application to a typical television videomonitor. Referring briefly to Figure 6, there is shown a typical videosignal having synchronizing (sync) pulses, back porch reference, videoblack level reference, and video signal. The upper wave form differsfrom the lower waveform only in the amplitude of the video signal. Sincethe illustrative signals are used for calibration, the black cal signalis of smaller amplitude than the white cal signal. As can be seen inFIG. 3, the write intensity feedback loop actually comprises twoseparate loops, one for detecting the brightest output of the writingdevice 20 and one for detecting the minimum brightness of the writingdevice 20. For purposes of this description, it will be assumed that thewriting device 20 comprises a CRT. As will be understood, thecalibration of the output of the writing device 20 can be done on aperiodic basis between film exposures. In order to achieve thisfunction, the image data is coupled into a sync pulse separator 36 whichdetects the synchronizing pulses contained in the image data signal andprovides appropriate timing signals to various sections of thecalibration and feedback control loops to insure that the calibrationcycles occur at appropriate times. Such synchronizing circuits are wellknown in the video processing art and will not be described herein.

Referring first to the black level control loop which establishes theintensity of the minimum light output of the writing device 20, there isprovided a photosensor 38 positioned adjacent the CRT 20 which providesan output signal to a black sample gate 40. The output signal from thephotodetector 38 is proportional to the light output of the CRT 20.However, during the time that the output of the black sensor 38 issampled, the system is set to produce a minimum gray scale or minimumlight output signal at the point adjacent the photodetector 38. Forexample, the detector 38 typically samples only a single scan line onthe face of a CRT at any one instant. Consequently, the gate 40 needonly synchronize sampling to a time at which a scan line appears infront of detector 38. The black sample gate 40 is an electronic samplinggate of a type well known in the art and may comprise for example, agated digital register or an FET switch connected to supply the sampledsignal to a storage capacitor. The signal sampled by the black samplegate 40 is applied to a comparator 42 in which it is compared to a blackreference signal established by a black reference potentiometer 44. Byadjusting the potentiometer 44, the black level or minimum gray scalevalue of the CRT 20 can be adjusted. The comparator 42 produces an errorsignal which is applied to a black sample storage circuit 46, whichcircuit 46 may include a capacitor storage circuit having sufficientcapability to hold a particular voltage level signal at a substantiallyconstant value between calibration times. The circuit 46 acts as aproportional plus integral circuit whose output value is incremented ordecremented by the signal from comparator 42 in order to generate anappropriate black level control signal. It will be noted that both theblack sample gate 40 and the black sample store circuit 46 are operatedby a gating signal supplied by an auto-density control circuit 48. Thecontrol circuit 48 is in turn provided with a synchronizing signal fromthe sync separator 36. The control circuit 48 provides the appropriatereference signal V_(R) ' for driving the CRT 20 during the calibratingcycle and also provides the timing signals to allow the black samplegate 40 and black sample store circuit 46 to sample the signal from thephotodetector 38 at the appropriate time. The control circuit 48 alsosupplies the signals for the white sample control loop which controlsthe maximum write intensity of the writing device 20.

Continuing with the black level control loop, the black sample signalstored in circuit 46 si coupled through an amplifier 50 to a clampcircuit 52. The clamp circuit 52 is driven also by the sync separator 36which provides a control signal to the black control amplifier 54 at apredetermined time. More particularly, the clamp 52 assures that theblack level for the video image data is set only when the minimum blackvideo signal is present. The adjustment of amplifiers for drivingcathode ray tubes and in particular the setting of black levels is wellknown in the television art and will not be described herein.

The setting of the maximum intensity level of light output of thewriting device 20 is very similar to the setting of the black level. Awhite sensor 56 is positioned adjacent the face of the CRT 20 andprovides an output signal to a white sample gate 58 which is clocked bythe control circuit 48 to sample the output (maximum white level) of thephototransistor white sensor 56 at the appropriate time. The sampledoutput of the white sensor 56 is compared with a white reference signalin a comparator 60. The resulting signal from comparator 60 is coupledto a white sample store circuit 62 which is also gated by the controlcircuit 48 at appropriate times. An output signal developed by the whitesample store circuit 62 is coupled through an amplifier 64 to anautomatic contrast control circuit 66. The auto contrast control circuit66 may be of the type typically used in television receivers. Inessence, the auto contrast control 66 is a variable gain amplifier whichadjusts the amplitude of the image data applied to it in a manner tocompensate for any deviations from the desired maximum intensity at theCRT 20. The video amplifier 68 couples the video and calibration signalsto the CRT and is similarly a standard video amplifier of a type wellknown in the art.

Although the black level, actually the minimum desired gray scale, isset first, it will be appreciated that the setting or adjustingprocedure must be repeated after the white level is set since the whiteadjustment controls the gain of the amplifier 66 which will vary theamplitude of the entire video signal including that portioncorresponding to video black which is used to clamp the black level.Accordingly, the control circuit 48 is adapted to reiterate the blacklevel set procedure after the white level is set.

It will be noted that a sensitometer generator 70 produces theaforementioned calibrated sensitometer output signal utilized togenerate the sensitometric strip 32 on the film 30 as shown in FIG. 2.The sensitometer generator 70 is also provided with a synchronizingsignal from the sync separator 36. A density calibration circuit 72which is manually activated by a density calibration input signal froman operator, is utilized to activate a switch 74 during the densitycalibration mode. The density calibration circuit 72 provides a signalto cause a switch 74 to switch from the video image terminal 10 to asensitometer generator terminal 76. During the density calibrationcycle, the auto density circuit 48 opens the switch 16 to disconnect theimage data to allow the density calibration circuit 72 to supply a knownreference signal to the write control system. The signal from densitycalibration circuit 72 is supplied through a summing junction 78 to aninput terminal of the auto contrast control circuit 66. The auto densitycircuit 48 also has control of the electronic switch 80 to move it tobypass the auto contrast control circuit 66 during the black levelrestoration process. As will be appreciated, the auto contrast controlwould attempt to compensate for low light levels if the switch 80 wereleft connected to the contrast control circuit 66 during initialcalibration. During the reiterated black level calibration steps, theswitch 80 is left in the position such that the video signal passesthrough the amplifier 66 so that the black level clamp voltage includesthe gain of amplifier 66 to compensate for the gain shift of actual graylevel above the back porch level.

Although the photodetectors or sensors 38 and 56 have been illustratedas two separate sensors, it will be appreciated that a single sensorcould be utilized by sequential reading of black and white levels. Inthe system in which both sensors are used, the black and white sensorscan be read in parallel by appropriately controlling the drive signalsto the CRT so that white or maximimum brightness is attained adjacentthe white level sensor and black or minimum brightness is attainedadjacent the black or gray level sensor. In either case, the autodensity circuit 48 provides a synchronizing signal to the sample gates40 and 58 to latch the data obtained from the black and white sensors.The sampled black level signal from gate 40 is compared in thecomparator 42 with the reference signal on a repetitive cycle until thedesired black level is achieved. The white sample circuit 62 is thenoperated to adjust the white level signal until the white level reachesa desired magnitude. The black level can then be re-checked and theprocess iterated until both white and black levels are at the desiredlevel or until the error signals produced by the comparators 42 and 60are sufficiently small.

Once the write intensity of the film writing device 20 has been set to acalibrated level, the film density loop can be activated to permit thegain of the first transfer device 14 to be adjusted to a level tocompensate for any deviations from desired film density. It should benoted that calibration of the write intensity output of the film writingdevice 20 does not require that film be exposed. A shutter can beprovided and kept closed during the write intensity calibration process.

FIG. 4 is another embodiment of the present invention in which the filmdensity control loop is incorporated in the write intensity controlloop. Specifically, the black sample store circuit 46 and the whitesample store circuit 62 are each replaced by a density control circuit82 and 84, respectively. The circuits 82 and 84 function in the samemanner as the circuits 46 and 62 but additionally include a directcurrent (DC) level shift capability which is responsive to a densitycontrol signal for shifting the DC level of their respective outputsignals. The black level control loop is responsive to a minimiumdensity control signal while the white level control loop is responsiveto a maximum density control signal.

In order to clarify the operation of the system of FIG. 4, reference isnow made to FIG. 5 in which there is shown a block diagram of apparatusfor generating the minimum and maximum density control signals. Theblock 86 labeled auto-density calibration controller is a part of theblock 48 of the system block diagram. The block 86 is describedseparately here inasmuch as the prior description has already disclosedthe various other functions which signals from the auto-density controlcircuit 48 perform. It will be noted that control circuit 48 is labeledas an auto-brightness controller in FIG. 4 even though it is essentiallythe same circuit 48 as described with respect to FIG. 3, theauto-brightness label is applied here since the circuit functions inconjunction with the brightness or intensity control loop for the CRT20. As shown in FIG. 5, the auto-density calibration command which isgenerally a manual operation command initiates the auto-densitycalibration cycle. The manual initiation is required since thecalibration cycle requires that an exposed calibration cassette such asthat illustrated at 88 or an exposed portion of a film be placed in aposition to be analyzed by the photodetector associated with the system.The calibration cassette 88 is an alternate embodiment of the previouslydescribed calibration film strip using the sensitomer strips andcalibrated sensitometer blocks. In the calibration cassette, tworeference film densities 90 and 92, are provided to enable the photodetector 94 to become calibrated to particular maximum density andminimum density levels represented by the two reference levels. A photodetector positioner 96 which may be a motor driven positioner ismechanically coupled to the photo detector 94 and arranged to positionit at locations wherein light passing through the maximum and minimumfilm density reference blocks 90 and 92 can be sampled by the photodetector 94. Actuation of the detector positioner 96 is by a signalcoupled from the auto density calibration controller 86. Operation ofthe detector positioner may be either in a timed sequence or there maybe provided feedback signals from the detector 96 to identify theparticular location of the detector as it moves the photo detector 94 bythe reference film strips. The photo detector also is driven past thesensitometer strip 98 which is the same type of sensitometer stripdescribed previously with regard to FIG. 2. More particularly, thesensitometer strip is exposed along an edge of an X-ray film orcalibration cassette by light emitted from the CRT or laser device 20.Thus, as the photo detector passes along the sensitometer strip 98, itcan pick out those particular blocks within that strip which correspondto the desired maximum and minimum density values represented by theblocks 90 and 92. It will be appreciated of course, that the photodetector does not actually do the comparison but does provide thesignals which are coupled through an amplifier 100 to the circuitrywhich actually compares the maximum and minimum density values to thevalues developed along the edge of the film in the sensitometer strip98.

The signals developed by the amplifier 100 are coupled through an analogto digital converter 102 to a reference register 104 and a sampleregister 106. The values of the maximum and minimum density strips 90and 92 are stored in the reference register 104 while the sampleregister 106 is used to store the values in digital form of each of thescaled density areas in the sensitometer strip 98. Clock signalsprovided from the auto density calibration controller 86 are used toclock the reference register 104 and sample register 106 to both allowthe sample values to be stored and also to further clock the values fromthe registers into a digital comparator 108.

The results of the digital comparison in comparator 108 are coupledthrough AND gates 110 and 112 to respective minimum density register 114and maximum density register 116. The AND gates 110 and 112 are gated orselected by signals supplied from the auto density calibrationcontroller 86. Obviously, if the comparator 108 is searching for aminimum density match, then the results of that comparison should bestored in the minimum register 114. Likewise, a maximum density matchwould be stored in the maximum density register 116. Thus, thecontroller 86 provides select signals to each of the AND circuits 110and 112 in order to gate the appropriate AND circuits to enable theproper register 114 or 116 to receive the step code information fromcontroller 86. The minimum density register 114 has its output connectedto a look-up table 118 which is used to convert the density step code toan appropriate gain control or DC offset value, in this instance aminimum density control value, which is supplied to the black leveldensity control block 82 in FIG. 4. Similarly, the density step codefrom register 116 is also applied to a look-up table 120 in which it isconverted to a maximum density control signal supplied to the densitycontrol block 84 in FIG. 4.

It will be apparent that the various control signals from the autodensity calibration controller are necessary in order to synchronize theoperation of the photo detector 94 and to appropriately compare each ofthe gray scale values sequentially in the sensitometer stripe 98 to theappropriate maximum or minimum reference value 90 or 92 in order todetermine the appropriate density control value to be applied to thecircuit 82 and 84.

The automatic brightness control of the present invention may beinitiated automatically or by command from an X-ray machine operator.The brightness calibration sequence corrects for any change inbrightness and/or offsets which are due to component changes with timeand temperature. When the automatic brightness command is received, thecontroller first opens the switch 16 to isolate the video input signalfrom the control system. The shutter between the CRT and the X-ray filmis normally closed at this point to prevent exposing the film during thebrightness calibration sequence. The auto brightness circuit thenchanges the position of switch 80 and begins the black level setprocedure by generating a black sample signal at the same time that thecalibration pulses are supplied to the black control circuit 54.Changing the position of the switch 80 by-passes the automatic contrastcontrol or variable gain amplifier 66 which would defeat the initialsetting of the black level control. After the black level control loophas provided a satisfactory minimum black loop error out of thecomparator 42, the switch 80 is released back to its normal position andthe white calibration signal and white samples pulses are generated toinitiate the white level calibration sequence. When the white level looperror is determined to be at a desirable minimum level, the controllergoes back to repeat the black loop setting with the auto contrastcontrol 66 in the system. As was mentioned previously, this is necessarysince the gain of the auto contrast control 66 will affect the level ofthe minimum video or black level signal. The system will reiterate theblack and white level control loop calibration cycles until the looperror in each loop is within a minimum range. In the embodiment shown inFIG. 4, the maximum density and minimum density control signals are setat zero during the brightness control calibration cycles since it isnecessary to isolate the density control offset values during thebrightness calibration cycles.

In order to automatically calibrate the system for film densityvariations, it is necessary to process a film using appropriatecalibration signals as described above to generate the sensitometrystrips necessary to allow the system to match desired reference valueswith actual measured values. In the preferred embodiment illustrated inFIG. 5, the film is processed and developed and then placed in thespecial calibration cassette holder 88. The calibration cassette 88 withthe test film is loaded in the system and an auto density calibrationcommand is activated to begin the auto density calibration process. Theauto density controller 86 illuminates the test film with flat whitelight from the CRT. The detector 94 is positioned to read the minimumdensity reference shown in the film strip 92 and the value of thatreference is then temporarily stored in the reference register 104. Thedetector is then moved down to view the exposed film sample strip 98 tosearch for an equivalent density level on the test film. When thecomparator 108 detects a match, the controller outputs a step code whichis stored in the register 114. The minimum density step code is a codevalue which is available in the auto density calibration controller 86as it positions the photo sensor 97 in search of the step on film strip98 which matches the reference density value. More particularly, thefilm density strip 98 is generated by a sequence of step voltage valueswhich are incremented across the CRT during the calibration cycle. Thesevalues expose the film creating a sequence of steps varying from belowthe minimum reference density value to above the maximum referencedensity value. The controlled sequences the step values in the orderthey appear on the film strip. Accordingly, when the comparator detectsa match between a particular step value or film density value and thereference value, the controller then picks from its memory theparticular voltage or density value which created that particulardensity value on the film. That value is then stored in the minimumdensity register. Similarly, once the minimum density value has beenfound, the photo detector then moves into position to store the valueassociated with the maximum density reference at strip 90 and then movesdown to monitor each of the values in the film strip 98 until acorresponding maximum density value is isolated. When the comparator 108provides a matched signal to the controller 86, the controller thenprovides a step code to the maximum density step register correspondingto that particular value of film density. The register then outputs thatvalue to a look-up table which provides the appropriate gain controlsignal to the white level density control loop.

The sensitometer signal generator 70 generates a conventional stair stepsignal which is identical to the normal input video in amplitude and setup. This signal is used to expose a piece of film as a periodicverification of the performance of the system. The process of generatingthe stair step voltage and providing a sensitometer strip are well knownin the X-ray film art.

It should now be appreciated that this invention provides a method andapparatus for automatically controlling the processing of electronicimages to film images with compensation for variations in the lightoutput of a film writing device and variations in the film and filmprocessing device.

We claim:
 1. In an imaging system for generating an image on film fromvideo image data, the system including at least one controllabletransfer device for controlling the image data, a method for controllingfilm density comprising the steps of:providing a film writing devicehaving a predetermined light intensity output in response to apredetermined input command signal amplitude; generating an inputcommand signal having an amplitude varying as a function of position ofa light beam from the film writing device whereby the light intensityoutput varies in correspondence with beam position; exposing a film tothe light output from the film writing device; developing the exposedfilm; reading the image density on the film using the light output fromthe film writing device at the predetermined intensity; and adjustingthe controllable transfer device in response to the image intensity readon the film to effect a corresponding adjustment in the image data tocorrect for deviations in film density from a desired value.
 2. Themethod of claim 1 wherein said step of reading the image density of thefilm comprises the further steps of:generating a light beam of uniformintensity from the film writing device; scanning the generated lightbeam through a neutral density sensitometric block; detecting the lightbeam passing through the sensitometric block with a photodetector;calibrating the photodetector to produce output signals corresponding tothe grey scale values of the sensitometric block based upon the lightbeam passing therethrough; scanning the generated light beam through theexposed film; and reading the film density with the calibratedphotodetector.
 3. The method of claim 2 wherein the output signalsdeveloped by the photodetector during the film reading step are appliedto the transfer device to modify its transfer characteristic tocompensate for film density deviations.
 4. The method of claim 1 whereinsaid step of providing a film writing device having a predeterminedlight intensity output comprises the substeps of:driving the filmwriting device with a signal having a predetermined value through afirst controllable gain amplifier; measuring the light intensity outputof the film writing device; and adjusting the gain of the firstamplifier to force the predetermined light intensity output.
 5. Themethod of claim 4 and including the further steps of:measuring theminimum light intensity output of the film writing device; and settingthe amplitude of bias voltage applied to the device such that a desiredminimum light intensity is achieved.
 6. A control system for providingsubstantially constant film density of negative and positive images onfilm in a system for developing film images from electronic image data,the control system comprising:a film writing device for providing acontrollable light intensity output in response to a write intensityinput signal; means for exposing the film to the light output of saiddevice; means for developing the exposed film; means for determining thefilm density of the developed film and for generating a signalrepresentative of the film density; and means responsive to said filmdensity representative signal for varying the write intensity inputsignal in a manner to adjust the light intensity output to compensatefor deviations in film density from a desired level.
 7. The controlsystem of claim 6 wherein said film writing device comprises:acontrollable light source; a variable gain amplifier having an inputterminal for receiving the write intensity input signal and an outputterminal for providing a drive signal to said light source, and furtherincluding a first control terminal for receiving a gain control signal;means for monitoring the light intensity output of said light source,said monitoring means providing a signal representative of the intensityof light output from said light source; and means coupling said lightintensity representative signal to said amplifier control terminal forselectively varying the gain of said amplifier in a direction to providea predetermined light intensity output in response to a predeterminedmagnitude of input signal.
 8. The control system of claim 6 wherein saidmeans responsive to said film density representative signal comprises acontrollable transfer device having a data input signal terminal, a dataoutput terminal and a control terminal, said transfer device operatingon the image data applied to said input terminal to provide an outputsignal at said data output terminal having a determinable relationshipto the image data, said determinable relationship being variable inresponse to said film density representative signal applied to saidcontrol terminal.
 9. The control system of claim 8 wherein said transferdevice comprises a variable gain analog amplifier.
 10. The controlsystem of claim 8 wherein said transfer device comprises a digitalmemory device having a plurality of light intensity values stored in alook-up table format, the image data being formatted as an address toeffect readout of selected stored values in response to thecorresponding image data.
 11. The control system of claim 7 wherein saidamplifier includes a second control terminal, said amplifier beingarranged to have a variable zero offset in response to a signal appliedto said second control terminal, said control system including:means fordetermining minimum light intensity output of said film writing deviceand for producing a signal representative thereof; and means forcoupling said minimum intensity signal to said second control terminalwhereby said amplifier offset is adjusted to provide a desired minimumlight intensity.
 12. The control system of claim 11 wherein saidamplifier comprises:a first variable gain linear amplifier providing anoutput signal proportional to the write intensity input signal, theproportional relationship being variable in response to said lightintensity representative signal; and a second linear amplifier having apredetermined fixed gain and an adjustable minimum output signal, saidsecond amplifier including said second control terminal for effectingadjustment of said minimum output signal.
 13. The control system ofclaim 12 wherein said means for monitoring the light intensity outputcomprises a light sensing device connected in a light path of said lightsource, said light sensing device providing an output signalproportional to the light output of said light source adjacent saidlight sensing device.
 14. The control system of claim 13 wherein saidmeans coupling said light intensity representative signal to saidamplifier control terminal comprises:means for selectively sampling theoutput signal from said light sensing device; means for comparing saidsampled output signal to a predetermined reference signal and forproviding a summation signal representative of the differencetherebetween; means for storing said summation signal; and means forapplying said stored summation signal to said first control terminal.15. The control system of claim 11 wherein said means for determiningminimum light intensity output comprises a light sensing deviceconnected in a light path of said light source, said light sensingdevice providing an output signal proportional to the light output ofsaid light source adjacent said light sensing device.
 16. The controlsystem of claim 15 wherein said means for coupling said minimumintensity signal comprises:means for selectively sampling the outputsignal from said light sensing device; means for comparing said sampledsignal to a predetermined reference signal and for providing a summationsignal representative of the difference therebetween; means for storingsaid summation signal; and means for applying said stored summationsignal to said second control terminal.
 17. The control system of claim16 wherein said means for applying said stored summation signalcomprises a clamp circuit synchronized to a standard video wave formsuch that said minimum intensity level is set only when said videosignal is at corresponding minimum level.
 18. The control system ofclaim 7 wherein said coupling means includes said film densityresponsive means.
 19. The control system of claim 18 wherein said filmdensity responsive means comprises a variable offset proportional plusintegral signal transfer device having a signal input terminal forreceiving a signal representative of the difference between an intensityreference signal and said light intensity representative signal andfurther including a control terminal for receiving said film densityrepresentative signal for varying said difference signal in proportionthereto.
 20. The control system of claim 7 wherein said monitoring meansincludes a first photodetecting sensor for monitoring light output ofsaid light source at a first minimum level and a second photodetectingsensor for monitoring light output of said light source at a secondmaximum level.
 21. The control system of claim 7 wherein said couplingmeans includes a first control loop for adjusting the minimum lightintensity output and a second control loop for adjusting the maximumlight intensity output.